Digital design component with scan clock generation

ABSTRACT

A master and a slave stage of a flip-flop are each separately clocked with non-overlapping clock signals during scan mode to eliminate a data input scan mode multiplexer. Separate, non-overlapping clocking permits the elimination of hold violations in scan mode for scan mode flip flop chains, permitting the elimination of delay buffers in the scan mode data paths. Resulting application circuits have reduced circuit area, power consumption and noise generation. A clock generator for scan mode clocking is provided to obtain the separate, non-overlapping scan mode clocks. Scan mode clocks may be generated with a toggle flip flop, a pulse generator or a clock gating circuit.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

N/A

BACKGROUND OF THE INVENTION

1. The present invention relates generally to a digital design component used in the design of high level digital circuits, and relates more particularly to a flip-flop design element and scan clock generation therefor.

Description of Related Art

In the implementation of digital design systems or application specific integrated circuits (ASIC), one important criteria is timing closure associated with the various interconnected design components. High performance designs benefit from increased implementation efficiency to obtain higher frequency targets in a resulting circuit. High frequency operation implies the use of fewer gates between flip-flops and an increasing use of pipelining techniques. Pipeline techniques permit higher operating frequencies by breaking up critical paths into one or more states separated by flip-flops. The use of pipelining techniques typically increases the number of sequential cells used in a design. Accordingly, the realization of high frequency designs focuses on providing more efficient sequential cells that allow rapid timing closure during chip implementation.

A traditional multiplexed scan flip-flop design has a number of characteristic features associated with the system level implementation of the design. The efficiency of sequential cells is traditionally measured through observation of parameters such as data set up time, clock to “Q” delay, data hold time and cell area. The operating frequency of a chip is typically limited by intrinsic delays, setup times and tolerance variations in clock duty cycles. One measure to determine the limitations on the chip operating frequency is to observe the minimum operating clock as a sum of delays, setup times and tolerance variations. The following equation provides one measure of determining a minimum operating clock. TCLK_MIN=ΔT _(CLK) _(—) _(Q) +ΔTg+Δtsetup+CLK _(—) SKEW+CLK_JITTER  (1)

Where:

-   -   ΔT_(CLK) _(—) _(Q)=flip-flop clock-to-Q output delay     -   ΔTg=critical path gate delays plus RC     -   ΔTsetup=flip-flop data to clock setup time     -   CLK_SKEW=the variation in clock tree insertion delay and OCV         induced clock insertion delay differences     -   CLK_JITTER=duty cycle variations (cycle to cycle).

Traditional sequential cell design focuses on optimization of cell area, ΔT_(CLK) _(—) _(Q) and ΔTsetup. However, at a system level, optimization of the clock period focuses on the minimization of each term in the minimum clock period of equation (1).

Referring to FIG. 1, multiplexer 11 is provided at an input to the flip-flop to permit selection between functional data D and scan data SD. The presence of multiplexer 11 has an impact on the ΔTg term, which can be viewed from different perspectives with respect to critical path optimization. First, multiplexer 11 may be viewed as increasing the critical data path delay of the functional mode data. Second, multiplexer 11 may be viewed as consuming one gate delay of critical paths gate delay budget. Third, multiplexer 11 may be viewed as increasing the setup time for the flip-flop. In either case, optimization of the minimum clock period is influenced by the presence of multiplexer 11. In addition, multiplexer 11 increases power consumed in functional mode when both the functional and scan data input toggle when the flip-flop changes logic states.

Another difficulty with latched based, edge triggered flip-flops is observed in the potential race conditions when the same the clock edge is used to both launch and capture data. When the ΔT_(CLK) _(—) _(Q) and ΔTsetup delay values are minimized to increase operating frequency, there is an increased probability that two back-to-back flops can experience data race-through problems. Race-through occurs whenever an upstream flip-flop launches data before the downstream flip-flop stops capturing data. This effect is observed as a data hold time violation on the data input of the downstream input of the flip-flop. In addition, clock skew between launching and capturing flops often creates and/or exacerbates hold time violations. Furthermore, scan chain reordering is often conducted to reduce routing congestion in the chip. However, scan chain reordering has the potential to create a large set of scan mode hold violations, since scan data would then be routed to the closest possible flip-flop. The shorter routes lead to less propagation delay which in turn lead to higher likelihood of scan mode hold violations.

While hold violations can occur in any functional operating mode of the circuit, the greatest number of violations typically occur during scan shift/capture modes. The number of hold violations induced by scan chain reordering can be potentially enormous. Hold violations are usually fixed with the insertion of delay buffers in the path containing the hold violation. The inserted delay buffers are cells that are specially designed to have a greater than normal intrinsic cell delay. Accordingly, the delay buffers are typically inserted through design software, or automatically, in the hold violation path prior to the flip-flop that has the hold violation. Typically, the insertion of the delay buffers occurs immediately before the hold violation flip-flop. When the delay buffers are inserted before the receiving flip-flop, all the timing arcs that terminate in the data input port of the flip-flop with the hold violation are delayed. Accordingly, the introduction of delay buffers to overcome hold time violations has an impact on the entire system.

The drawbacks that typically occur with the insertion of delay buffers include additional usage of chip area, an increase in routing congestion, diminished signal integrity through increased cross talk and increased power consumption. Any of these drawbacks also can trigger additional implementation or timing closure iterations that add to the implementation costs of the circuit. The addition of a large number of hold buffers results in an increase in chip area.

Scan logic consumes a certain amount of power during normal functional mode operation, and circuit designs typically take into account routing and connectivity of scan logic paths and their impact on critical paths and operation of the normal functional mode logic. Conventionally, flip-flops with a multiplexed scan inputs often reuse the primary flip-flop data output such as “Q” and/or “QB” to propagate the scan data input to the next flip-flop in the scan logic chain. The reused outputs often result in metal routing and buffers in the scan data path that create parasitic loads on the critical paths that can impact critical path delays in the circuit. One technique to overcome parasitic loading, calls for a dedicated scan output in the flip-flop architecture. Such a technique is implementation sensitive and a ΔT_(CLK) _(—) _(Q) delay may be difficult to avoid in the critical path of the flip-flop. Scan logic implemented in a conventional flip-flop also typically has metal routing that is associated with the slave stage of the flip-flop to take advantage of some of the architectural features of the flip-flop. Accordingly, conventional scan logic draws power in the flip-flop operation during normal operating mode because the scan logic path toggles when the slave stage of the flip-flop changes state. Power consumption in the scan logic is also observed with a series of flip-flops where the scan logic path switches with every change in an upstream flip-flop state.

FIG. 2 illustrates a typical scan logic path containing a scan signal regeneration buffer 20 and a scan path delay buffer 22 connected to a scan multiplexer input 21 on a downstream scan flip-flop 24. The critical path is impacted by the scan logic as illustrated by ΔT_(g) delay 26. When upstream flip-flop 28 changes state, the scan path logic switches in relation to down stream flip-flop 24 thereby consuming additional power.

It would be desirable to obtain a fundamental flip-flop architecture that can improve a system level circuit with respect to reducing or eliminating hold violations impacting critical path delay and power consumption. It would also be desirable to obtain a system and method for generating scan clock signals for rapid, efficient and reliable scan operations during a scan mode.

BRIEF SUMMARY OF THE INVENTION

In accordance with the present invention, a flip-flop architecture with scan mode functionality eliminates scan mode hold violations and reduces chip area consumed, power consumed, noise generated and critical path delays for application architectures using the flip-flop design. The elimination of hold violations in scan mode permits the removal or omission of scan mode path delay buffers. The separation of various functions according to different modes in the architecture permits scan mode logic and functional mode logic to be independent so that operation of one mode path does not induce toggling or power consumption in another mode path.

The flip-flop architecture has a master-slave configuration to permit independent capture and output of data. In functional mode, each of these stages is clocked with the same functional mode clock. In scan mode, however, the master and slave stages are clocked separately. During scan mode, a scan mode clock provides the master stage clocking and the functional mode clock provides the slave stage clocking. The two clocks are non-overlapping, so that the duty cycle of both clocks together is less than a full clock period. Each clock signal has pulses that coincide with low states in the other clock signal. For example, both clocks may have duty cycles that together add up to less than 100%, such as less than 80%-20% or 50%-50%, respectively. The arrangement of non-overlapping clocks avoids the master latch and the slave latch being transparent, or passing data, at the same time. The functional mode clock is disabled with respect to the master stage during scan mode, and provided to the slave stage for output clocking in scan mode. The scan mode clock is enabled and applied to the master stage during scan mode, and otherwise inactive in functional mode. A feedback clocked inverter in the master stage is disabled by whichever clock is active, the functional clock or the scan clock.

The slave stage of the flip-flop architecture is provided with a gated scan mode output to permit independent scan data propagation. Accordingly, the scan data output is not active during functional mode so that scan logic is not toggled during functional mode. The separate, non-overlapping clocks used during scan mode eliminate scan mode hold violations in the flip-flop scan chain. Accordingly, the flip-flop architecture of the present invention permits savings in circuit area through the omission of scan mode delay buffers. A main difference between the flip-flop architecture of the present invention and that of prior designs is the additional scan mode clock input used in the flip-flop design of the present invention. The provision of the additional clock to the flip-flop permits significant advantages in flip-flop architecture and overall system design, as discussed above, and in greater detail below. In addition, the scan mode clocks may be generated according to a number of techniques.

According to an embodiment of the present invention, there is provided a flip-flop architecture including a master stage for receiving an input and a slave stage for proving an output. A plurality of clocking signals is selectively applied to the master stage to clock the master stage. A selection signal is provided to the flip-flop and applied to select one or more of the clocking signals. Different clocking signals may be applied to the master stage in dependence on a state of the selection signal.

According to another embodiment of the present invention, there is provided a flip-flop architecture with a scan mode data path clocked by a scan mode clock. A functional clock clocks a functional data path. The flip-flop includes a master stage including a clocked inverter selectively clocked with the functional mode clock or the scan mode clock. A slave stage is coupled to the master stage and clocked with the functional mode clock. A selection signal coupled to at least one of the clock signals selectively disables the clock signal to the master stage.

In accordance with another embodiment of the present invention, there is provided a method for operating a flip-flop. The method includes selecting a test mode for testing flip-flop operation and clocking a master stage of the flip-flop with a first clock signal during test mode. The method provides for selecting a normal functional mode for normal functional operation of the flip-flop and clocking the master stage of the flip-flop with a second clock signal during normal mode operation.

According to an embodiment of the present invention, non-overlapping scan mode clocks are generated with a clock gating system that selectively gates a clock signal to a desired destination. A system clock and a test clock are gated with test mode criteria to obtain a gated clock used to operate the slave stage of a series of flip-flops in scan mode, and a scan clock used to operate the master stage of the flip-flop according to the present invention in scan mode.

According to another embodiment of the present invention, a gated clock is provided to a scan clock pulse generator that provides the scan clock that operates the master stage in scan mode. The gated clock is based on a system clock or test clock, dependent upon test parameter signals that determine clock selection.

According to another embodiment of the present invention, non-overlapping scan clocks are generated using an input clock to create an event that is synchronized to a high-speed reference clock. In an exemplary embodiment, the high-speed reference clock operates four times faster than a given scan clock. It should be apparent that whole or fractional multiples of the scan clock speed may be used in the formation of the high-speed reference clock. The reference clock may be created with an on-chip Phase Locked Loop (PLL) or supplied externally, and need not be synchronized with the input clock. The event is used to cause a state machine to operate that allows a single pulse of the higher speed clock to propagate to a first scan clock, followed by one or more low cycles. The state machine then permits a single pulse to be presented to a second scan clock, followed by one or more low cycles. In this exemplary embodiment, there are four high-speed clock pulses for every pulse of the scan clocks. Scan clock pulses can be generated on any of the reference clock edges, as long as they are non-overlapping. For example, a first scan clock can be generated based on the third rising clock edge of the reference clock, while a second scan clock can be generated on the first rising clock edge.

According to another embodiment of the present invention, the non-overlapping scan clocks are generated from a gated clock, with one scan clock used for the slave stage, and one clock used for the master stage during scan mode in the flip-flop according to the present invention. The non-overlapping scan mode clocks can be configured to permit a large degree of tolerance in the clock distribution, such as may be needed in the presence of a large amount of clock skew. According to a feature of the present invention, the non-overlapping scan mode clocks are generated with one pulse each per gated clock cycle. With this clocking scheme, scan mode testing can proceed at the test clock frequency.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of the present invention will become clear from the following detailed description when read in conjunction with the accompanying drawings in which:

FIG. 1 is a circuit block diagram of a conventional flip-flop element;

FIG. 2 is a circuit block diagram illustrating hold violation repairs and system delays;

FIGS. 3A-3C are is a circuit block diagrams of flip-flop architecture according to the present invention;

FIG. 4 is a circuit block diagram of a flip-flop element according to another embodiment of the present invention;

FIG. 5 is a circuit block diagram of a flip-flop element according to another embodiment or the present invention;

FIGS. 5A and 5B are circuit block diagrams of a flip-flop element according to the present invention illustrating reset and preset embodiments;

FIG. 6 is a circuit block diagram illustrating scan clock generation according to an embodiment of the present invention;

FIG. 7 is a circuit block diagram for scan clock generation according to another embodiment of the present invention;

FIG. 8 is a timing diagram illustrating scan mode operation and scan clock generation in accordance with an embodiment of the present invention;

FIG. 9 is a circuit block diagram for scan clock generation according to another embodiment of the present invention;

FIG. 10 is a circuit block diagram of a pulse generator element according to an embodiment of the present invention;

FIG. 11 is a timing diagram illustrating scan mode operation and scan clock generation in accordance with an embodiment of the present invention;

FIG. 12 is a circuit block diagram for scan clock generation according to another embodiment of the present invention;

FIG. 13 is a circuit block diagram of a pulse generator element according to an embodiment of the present invention;

FIG. 14 is a circuit block diagram for scan clock generation according to another embodiment of the present invention; and

FIG. 15 is a timing diagram illustrating scan mode operation and scan clock generation in accordance with another embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention integrates the functionality of a scan mode multi-plexer into the structure of the flip-flop with non-overlapping scan mode clocks. The new flip-flop structure uses two non-overlapping clock signals during scan mode. The two clock signals can be generated according to a number of different techniques.

The scan mode flip-flop function previously realized as a two to one multiplexer and a single port latch, is obtained with a dual port latch with integrated clock gate, thereby providing a number of significant advantages. One particular advantage is that the novel flip-flop structure according to the present invention has many of the same features when viewed externally as previous muxed scan flip-flop elements so that it can be readily used in traditional tools and architectures. That is, the flip-flop maintains compatibility and portability with design tools. One difference is an additional scan mode clock provided to the flip-flop, which can be readily obtained.

Referring now to FIG. 3A, a circuit block diagram of a positive edge triggered flip-flop according to the present invention is illustrated generally as circuit 30. The flip-flop of circuit 30 includes scan functionality for a scan test, as well as circuitry for operating as a positive edge triggered flip-flop in functional mode. As can be seen from the architecture of circuit 30, a scan and functional multiplexer 10, as illustrated in FIG. 1 is omitted from the input of the flip-flop configured according to circuit 30. The removal an input multiplexer for scan and functional modes provides a significant improvement in the operation of the flip-flop with respect to critical path delay and chip power consumption. The functional operations of input multiplexer 10 are incorporated into circuit 30 to remove scan logic from the critical path and eliminate scan hold violations.

NOR gate 32 is active with a scan enable signal SE to disable clocking of master latch 34. During scan mode, a scan clock SCK2 is used to clock master stage latch 34, with data provided on scan data input SD. Master clock signal CLK is used to clock slave latch 36 during functional or scan modes. In this arrangement, the flip-flop of circuit 30 is configured as back to back master and slave latches 34, 36. This arrangement permits non-overlapping master and slave scan clock operation during scan mode, and also maintains edge triggered operation during functional mode.

The non-overlapping scan clocks eliminate all scan mode hold violations. Without the scan mode hold violations typically encountered in conventional flip-flops, the flip-flop of circuit 30 eliminates any need to insert delay buffers, leading to a significant reduction in routing, cell area, and power consumption.

Clocking for the master stage during scan mode is provided by clocking signal SCK2. Scan mode enabled clocking is also incorporated into the feedback in the master stage latch with complementary CMOS transistors 37, 38, gated by clock signals SCK2 and SCK2X, respectively. When the flip-flop operates in functional mode, clock signals PH2 and PH2X drive CMOS transistors 27, 28 respectively, to provide master stage clocking. With this configuration, master stage 34 is operated with either the system clock in functional mode or the scan clock in scan mode. Accordingly, master stage 34 is operated with a selective clock signal in dependence upon the selection of functional or scan mode. This functionality is the same as that provided by a multiplexer, but avoids the critical path delays in the functional mode, reduces set-up time for the flip-flop and avoids toggled scan mode logic lines during functional mode.

Reconfiguring the flop into back to back latches that are enabled by non-overlapping clocks eliminates scan mode hold violations. Accordingly, delay buffers need not be inserted in the scan logic between flip-flop elements, thereby improving component density, reducing routing congestion, improving resistance to cross-talk interference and reducing power consumption. In addition, the elimination of scan mode hold violations eliminates the scan mode hold timing closure and a number of timing corners used to validate circuit design. In an exemplary embodiment, the elimination of scan mode hold violations removes 8 timing corners from a total of 96 used to validate the final circuit design, providing an 8.3% reduction in parameter validation. The reduction in timing corners for circuit validation also leads to a reduction in iterations for design configurations that would otherwise be used to repair the scan mode hold violations. The resulting improvement in chip implementation cycle time can be significant.

One area where chip implementation cycle time is greatly improved concerns the difficulty in scan chain reordering. Scan chain reordering is typically invoked to improve routing congestion in the design of the circuit using scan mode test and verification. However, when a scan chain is reordered to reduce routing congestion, scan mode connections are typically routed through the closest available flip-flops, which represent very little delay in the scan data path. In previous scan mode implementations, the reduced delay resulting from scan chain reordering induced a large number of scan hold violations, which were typically repaired with delay buffers, adding to the implementation cycle and decreasing the efficiency of the resulting circuit. With the elimination of scan mode hold violations in accordance with the present invention, the drawbacks attendant with prior flip-flop configurations are eliminated.

Signal SQ in FIG. 3 represents the scan mode output of the flip-flop constructed according to circuit 30. A NAND gate 31 provides an enable gate for the scan output so that the scan output signal SQ is available during scan mode. Otherwise, in functional mode where scan logic is not enabled, scan output signal SQ is inactive due to the gating provided by NAND gate 31. The enablement of scan output signal SQ during scan mode prevents toggling of scan data path logic during functional operation, which significantly reduces power consumption of the chip incorporating circuit 30.

Circuit 30 provides a technique for testing the critical path of the implemented flip-flop while eliminating hold violations and significantly reducing power consumption and interference in the chip implementation. The cycle time for the chip implementation is also significantly reduced because of the greater simplicity provided in addressing practical considerations of chip design using circuit 30.

Referring now to FIG. 3B, a flip-flop architecture similar to that illustrated in FIG. 3A is shown as architecture 33. Architecture 33 is substantially similar to circuit 30, with the differences an additional inverter provided after the functional portion of the master stage to produce a non-inverted functional mode output. Similarly, the inverter used to produce the inverted functional mode output of circuit 30 is eliminated in architecture 33.

Referring now to FIG. 3C, another embodiment of the flip-flop architecture according to the present invention is illustrated as architecture 35. Architecture 35 provides a number of the same features as circuit 30 and architecture 33, but offers a simplified feedback in the scan mode master stage to obtain additional utilization of parallel features of functional mode and the scan mode elements. In this embodiment, scan clock SCK2 clocks the feedback mechanism of the scan mode master stage, while scan clock SCK1 provides the clocking for the buffer between the scan mode master stage and the slave stage of the flip-flop.

Referring now to FIG. 4, a positive edge triggered flip-flop with scan output gating is illustrated generally as circuit 40. Circuit 40 is essentially the same as circuit 30 with the additional implementation of a NAND gate 42 in the input stage of master latch 44. The insertion of NAND gate 42 illustrates the ability of circuit 40 to absorb additional simple Boolean functions into the input stage of master latch 44. The additional Boolean functions are available as a direct result of integrating the scan mode multiplexer into the flip-flop. The provision of simple Boolean functions on the front end of the flip-flop potentially permits a decrease in the number of external logic gates that may impact critical path delays.

Referring now to FIG. 5, an implementation of a negative edge triggered flip-flop with scan output gating is illustrated generally as circuit 50. Negative edge triggered flip-flops, such as that illustrated in circuit 50, are somewhat uncommon in typical application designs. Accordingly, the area consumed by a negative edge flip-flop is not as critical as that of a positive edge triggered flip-flop. This observation can be used to advantage in constructing a negative edge trigger flip-flop in accordance with the present invention to make the negative edge triggered flip-flop directly compatible with the positive edge triggered flip-flop functional and scan clocking methodology. That is, even though the negative edge triggered flip-flop produced according to the present invention may not be optimally efficient, it provides a direct compatibility with positive edge triggered flip-flops for scan mode. This compatibility avoids the need to incorporate lock-up latches on scan data inputs and outputs. Exclusive NOR (XNOR) gate 52 operates on slave stage 56 in conjunction with the functional clock and scan enable signal SE. XNOR gate 52 is non-inverting when scan enable signal SE is active, for example a “1”. During scan mode, when the functional clock is non-inverting on the slave stage, the resulting clock signal on the slave stage is a non-overlapping slave stage scan clock. When scan mode is disabled, for example during functional mode, XNOR gate 52 is inverting, providing a negative edge flip-flop functional operation. When functional data from a positive edge flip-flop is provided to a negative edge flip-flop, a negative clock sense latch is used on the data path. When functional data from a negative edge flip-flop is sent to a positive edge flip-flop, a positive clock sense latch is used on the data path. The specifications are similar to those of conventional flip-flops, and maintain a compatibility with test equipment and conventional elements.

The flip-flop of the present invention also permits asynchronous preset and reset flip-flop configurations that are highly efficient with respect to area and power consumed. There is also very little impact on flip-flop speed with the addition of the preset or reset functionality. For examples, referring to FIGS. 5A and 5B, three additional transistors are used to implement the reset and preset functions with the flip-flop configuration according to the present invention. With respect to the reset function, the associated structure is hazard free so that recovery and removal arcs need not be characterized and modeled for parametrie performance.

Referring now to FIG. 6, an illustration of a first embodiment for scan clock generation is shown using a toggle flop 63, 73. FIG. 8 shows a timing diagram applicable to the embodiments of FIGS. 6 and 7. Scan clocks are generated by controlling enables of clock gating modules in clock generation blocks. The scan clocks are generated by enabling alternating input clock pulses to generate the various scan clocks. In FIG. 6, diagram 60 illustrates clocking for a chain of flip-flops 65-67, of which flip-flops 65-67 are conventional and flip-flop 66 is composed according to the present invention. Clock generator block 62 provides for the generation of a scan clock signal SCK2, which supplies the scan clocking for flip-flop 66 as well as scan clock signal SCK1, which provides scan clocking for pre-existing or conventional flip-flops 65, 67 that have multiplexer inputs. The clock gating used in the scan clock control and generation in block 62 may be independent of any preexisting functional clock gating in the system. The generated clocks SCK1 and SCK2 in diagram 60 are non-overlapping scan clocks that have a large enough spacing, or non-overlap, to eliminate all hold time violations. As illustrated in FIG. 8, the spacing between the clock pulses to prevent overlap between the scan clocks has some built in tolerance that allows for sloppy distribution of scan clock signals SCLK1 and SCLK2, or some significant amount of skew.

In diagrams 60, 70 clock signal SCK1 is used as the flip-flop clock for flip-flops 65-67 in functional mode, and also clocks the slave stage of flip-flop 66 in scan mode. Clock signal SCK2 clocks the master stage of flip-flop 66 in scan mode. The non-overlapping master and slave scan clocks for flip-flop 66 during scan operation is the result of the provision of two separate clocks during scan mode. The architecture of flip-flop 66 permits the system to maintain edge triggered operation during functional mode when a single clock is used.

Generation of the non-overlapping master and slave scan clocks is provided by the enablement of alternating cycles of a general clock signal GCLK to generate clock signals SCK1 and SCK2. The signals used to enable the alternating cycles of the general clock signal GCLK may be obtained through a variety of methods. In one embodiment, signal SCK_EN_MODE illustrated in diagram 60 is used to select between two different modes. A first mode provides direct test control through the selection of signal SCK_EN using multiplexer 65, as illustrated in diagram 60. Signal SCK_EN transitions on every other pulse of clock signal GCLK during scan mode, for example, as illustrated in FIG. 8.

Another method to generate the enables for the clocks is through the use of an internal toggle flop 63. Toggle flop 63 uses clock signal GCLK as an input to provide a reduced frequency clock signal that permits a simple enable signal for gating every other clock cycle of clock signal GCLK. The output of toggle flop 63 provides an enable that is available for selection of alternating clock cycles to generate scan clock signals SCK1 and SCK2. Toggle flop 63 changes state every other clock cycle of clock signal GCLK, to provide the alternating enables for latch circuits 68, 69. Latch circuits 68, 69 use the same clock signal input GCLK, and are alternately enabled by logic signals derived from toggle flop 63 or input SCK_EN, depending which is enabled by mode selection signal SCK_EN_MODE. Accordingly, every other clock cycle of clock signal GCLK is directed to one of latch circuits 68, 69, based on the state of the output of toggle flop 63 or signal SCK_EN. The resulting sets of non-overlapping clock signals are applied to flip-flop 66 to alternately clock the master and slave stages to realize the advantages of the present invention. The alternating clocks have no impact on flip-flop 65, 67, since they each receive only one clock signal of the two alternating available clock signals. Accordingly, the clock generation technique illustrated in diagram 60 acts to split the general clock signal GCLK into two separate, non-overlapping clock signals that represent alternating clock cycles of general clock signal GCLK. The conventional flip-flops 65, 67 receive a clock signal that is essentially half the frequency of the general clock signal GCLK, with a correspondingly reduced duty cycle. That is, the apparent duty cycle of the clocks applied to conventional flip-flop 65, 67 is approximately 25% as compared to the general clock signal GCLK that has an apparent duty cycle of approximately 50%. The relationship of the various clock signals and signal SCK_EN is illustrated in FIG. 8.

According to another embodiment, multiplexer 65, signal SCK_EN_MODE and signal SCK_EN may be eliminated so that non-overlapping clock signals SCK1 and SCK2 are generated simply based on general clock signal GCLK and scan enable signal SE. In this instance, toggle flop 73 changes state every other clock cycle to provide and enable for latch circuits 78, 79 that change with every other cycle of general clock signal GCLK. Accordingly, flip-flop 76 constructed according to the present invention receives two non-overlapping clock signals to clock scan data from input SI to output SO. Flip-flops 75, 77 receive a single clock signal to shift scan data from input SI to output SO. As discussed above, the clock signal provided to flip-flop 75, 77 represents a frequency that is half of that of general clock signal GCLK and has an apparent duty cycle of approximately 25%. In the embodiment illustrated in diagram 70, the output of toggle flop 73 is substantially similar to the waveform of signal SCK_EN, illustrated in FIG. 8. Accordingly, the output of toggle flop 73 changes state with each clock cycle, producing an enable signal with a frequency of approximately half that of general clock signal GCLK, and with approximately a 50% duty cycle.

An advantage provided by the clock generation architecture illustrated in diagrams 60 and 70 is the control of the scan clock separation through control of the input clock period and duty cycle. The scan mode tester can be adjusted to optimize the clock period and duty cycle during scan mode to increase the scan mode clock frequency. That is, with reduced clock duty cycle the clock high time can be reduced to further increase clock frequency during scan mode. According to the clocking architecture, the frequency of general clock signal GCLK is twice the frequency used for clocking during scan mode.

Referring now to FIG. 9, another embodiment of a scan clock generator is illustrated as diagram 80. In diagram 80, clock generator block 81 illustrates how scan mode clock frequency for clock signals SCK2 may be improved with the use of a small, power efficient, pulse generator. Diagram 80 illustrates the generation of scan clock signal SCK2 using a rising edge of an input clock signal GCLK. A pulse generator 82 produces scan clock signal SCK2 based on the rising edge of clock signal GCLK. Scan clock signal SCK1 is simply the inverted signal of clock signal GCLK to provide scan mode slave stage clocking. The gating of the signal to scan clock signal SCK1 is controlled through a multiplexer 84, controlled with scan enable signal SE. Since clock signal SCK2 is generated from the rising edge of clock signal GCLK, and clock signal SCK1 is the inversion of clock signal GCLK, clock signals SCK1 and SCK2 are formed as synchronized non-overlapping scan clock signals. The relationship of the scan clock signals applied to flip-flops 85-87 are illustrated in the timing diagram of FIG. 11. In FIG. 11, clock signal SCK2 is illustrated as short pulses determined from the rising edge of clock signal GCLK which are active while the scan enable signal SE is active. Clock signal SCK1 is the inverse of clock signal GCLK, while scan enable signal SE is active. Clock signal SCK1 is substantially identical to clock signal GCLK when scan enable signal SE is inactive. According to this configuration, during scan mode, the master stage of flip-flop 86 is operated with a short pulse signal from clock signal SCK2 during scan mode, while conventional flip-flops 85, 87, and the slave stage of flip-flop 86 are operated with the longer pulse clock signal SCK1.

Pulse generator 82 may be constructed very simply because the duty cycle and edge positioning of scan clock SCK2 with respect to SCK1 is non-critical. Referring now to FIG. 10, a simple illustration of an embodiment of a pulse generator to form clock signal SCK2 is illustrated as circuit 110. In the embodiment illustrated by circuit 110, the pulse generator used to produce clock signal SCK2 has two inputs, scan enable signal SE and clock signal GCLK. Accordingly, circuit 110 illustrated in FIG. 10 may be a simplified form of pulse generator 82, which does not use scan enable signal SE as an input. Circuit 110 operates off of the rising edge of clock signal GCKL to produce an upward transition on clock signal SCK2 after delay 112. Clock signal SCK2, the output of AND gate 115, transitions high with a high transition on clock signal GCLK, since both inputs are temporarily active or high. Once the rising edge signal of clock signal GCLK propagates through delay 114 and inverter 116, the output of and gate 115 turns off, leading to a downward transition for clock signal SCK2. Using this simple pulse generator in circuit 110, the delay between the rising edge of clock signal GCLK and the rising edge of clock signal SCK2 can be set using delay 112, while the pulse width of clock signal of SCK2 can be set using delay 114.

The advantage to providing scan clock signal SCK2 as short pulses determined by rising edges of clock signal GCLK is that the clock frequency applied to flip-flops 85-87 during scan mode is the same frequency at which flip-flops 85-87 are clocked. Accordingly, there is no reduction in scan mode clock frequency as is the case in the previous embodiment, where each of the scan clocks SCK1 and SCK2 operate at half the frequency of the applied general clock signal GCLK.

Another technique for generating scan clocks SCK1 and SCK2 is through the use of a reference clock, synchronized to an input clock through an event. The reference clock may be generated on-chip using a phase lock loop (PLL). The PLL may be synchronized with the reference clock to be independent of clock signal GCLK. Alternately, or in addition, the reference clock may be provided externally during testing or during a scan enable mode. Such a reference clock could always be oscillating, and need not be synchronous with clock signal GCLK. That is, in the example of the PLL generated reference clock, the reference clock pulses can be asynchronous to the input scan clock and may be four to eight, or more, times faster than the input or test clock frequency. Preferably, the reference clock is at least 4 times faster than the input or test clock frequency. A higher frequency PLL output domain can be synchronized with either edge of an input or test clock signal. With this synchronization, a high frequency clock signal can be provided to scan clock SCK2 followed by a small delay and the output of another high frequency signal to scan clock SCK1. The delay can be provided by gating off PLL clock cycles between the provisions of clock pulses on scan clocks SCK2 and SCK1. This configuration provides non-overlapping scan clock signals SCK1 and SCK2 with small duty cycles and high frequency. As discussed above, these non-overlapping scan mode clock signals would be compatible with pre-existing multiplexer based scan mode flip-flops.

Referring now to FIG. 12, another embodiment of circuitry for a scan clock generation is illustrated as diagram 120. Diagram 120 illustrates the generation of scan clock signals SCK1 and SCK2 as short pulses based on rising edges of clock signal GCLK. Blocks 122, 123 are used to generate pulses for scan clock signals SCK1 and SCK2 respectively. Blocks 122 and 123 operate similarly to the pulse generator illustrated in FIG. 10, except that block 122 produces a short pulse when clock signal GCLK transitions from a high to a low state, while block 123 produces a short pulse when clock signal GCLK transitions from low to high states. The length of the pulses produced by either of blocks 122 and 123 can be controlled through manipulation of delay elements 124, 125 respectively. Through the use of pulse generating blocks 122, 123 non-overlapping scan clock signals can be generated for flip-flop 126 constructed according to the present invention. Because clock signals SCK1 and SCK2 are generated from the falling edge and rising edge of clock signal GCLK, respectively, the scan clock signals provide clock timing on flip-flop 126 that is the same frequency as clock signal GCLK. That is, two scan clock pulses are generated for every pulse of clock signal GCLK, which permits flip-flop 126 to be clocked at the same frequency as conventional flip-flops 125 and 127. During functional mode, block 128 selects clock signal GCLK to be applied to the usual clock input of flip-flop 126. That is, when scan enable signal SE is low, clock signal GCLK is enabled to the output of block 128 as functional clock signal MCLK1, which is input to the clock input of flip-flop 126. When scan enable signal SE is high, clock signal SCK1 is enabled to the output of block 128 to form scan clock signal MCLK1 that is applied to the clock input of flip-flop 126. According to this embodiment, pulse generation blocks 122, 123 are non-active during functional mode to reduce power consumption of the resulting clock generation circuitry.

Referring to FIG. 13, an embodiment of the pulse generation circuitry to form clock signals SCK1 and SCK2 is illustrated as block 130. Block 130 represents pulse generation blocks 122, 123 of FIG. 12 formed in a consolidated circuit. The configuration of block 130 permits the omission of an AND gate, while maintaining functionality used to realized scan clocks MCLK1 and SCK2 in scan mode, as illustrated in FIG. 12.

Referring now to FIG. 14, another illustration of an embodiment for scan clocks generation of scan clocks SCK1 and SCK2 is illustrated generally as diagram 140. In diagram 140, pulse generators 142 and 143 provide the scan clock signals SCK1 and SCK2. The scan clock generation illustrated in diagram 140 has the advantage of reusing the same pulse generator blocks illustrated in FIG. 10 to permit a potential reduction in the number of different components used to form the overall circuit that includes flip-flops 145-147. Input AND gates 141 act as enable circuits to gate clock signal GCLK to a low state when scan enable signal SE is inactive. With the output of AND gates 141 tied low when signal SE is inactive, scan clock signal SCK2 is low, or inactive, to permit functional mode clocking for flip-flops 145-147. Accordingly, scan clock signal SCK2 is low or inactive during functional mode. During functional mode, scan enable signal SE is inactive, and selects clock signal GCLK to pass through a multiplexor 144. Clock signal SCK1 becomes the same as clock signal GCLK, and scan clock signal SCK2 is inactive. Flip-flop 146 formed according to the present invention thus uses functional mode clock signal GCLK to drive the master stage storage node in functional mode.

Referring to FIG. 15, an illustration of the arrangement of scan clocks SCK1 and SCK2 is illustrated in a timing diagram. The rising edges of the pulses in scan clocks SCK1 and SCK2 are aligned with the falling and rising edges of clock signal GCLK, respectively. Scan clocks SCK1 and SCK2 are active when the scan enable signal SE is high, or scan mode is enabled. By generating scan clock signals SCK1 and SCK2 as short pulses to provide non-overlapping scan clock signals, the apparent transfer rate of information through a flip-flop according to the present invention is the same as conventional flip-flops, since two scan clock pulses are generated for every clock cycle of the input clock cycle GCLK.

The present invention provides a new flip-flop architecture based on separate gated clock signals for driving a master and slave stage during a scan test mode. The arrangement of gated, non-overlapping clocks permits the removal of an input multiplexer in the flip-flop architecture and eliminates scan mode hold violations in the application using the flip-flop architecture. The elimination of scan mode hold violations significantly reduces power consumption, circuit area and generated noise due to the omission of delay buffers previously used to correct scan mode hold violations and the removal of scan mode logic from critical paths in the flip-flop. The new architecture also provides a scan mode output enable so that scan mode logic is decouppled from functional mode logic during normal operation. The new architecture is compatible with tools and test equipment used in the development and test of conventional flip-flop architecture applications. The new architecture also reduces implementation time by removing a number of timing corners that are typically needed to validate an application design. Scan chain reordering also is not impacted by hold violations when using the new flip-flop architecture, leading to more efficient design and implementation of a given flip-flop application circuit.

Although the present invention has been described in relation to particular embodiments thereof, other variations and modifications and other uses will become apparent to those skilled in the art from the description. It is intended therefore, that the present invention not be limited by the specific disclosure herein, but to be given the full scope indicated by the appended claims. 

1. A flip-flop architecture, comprising: a master stage for receiving an input and a slave stage for proving an output; a plurality of clocking signals selectively applied to the master stage to clock the master stage; and a selection signal provided to the flip-flop and applied to select one or more of the clocking signals, whereby different clocking signals may be applied to the master stage in dependence on a state of the selection signal.
 2. The architecture according to claim 1, wherein the selection signal is coupled to one or more of the clocking signals applicable to the master stage to disable the one or more clocking signals when the selection signal is active.
 3. The architecture according to claim 1, further comprising an output enable, the output enable being driven by the selection signal.
 4. The architecture according to claim 1, wherein the master stage further comprises a clocked inverter operable based on the plurality of clocking signals.
 5. The architecture according to claim 4, wherein the clocked inverter is arranged to permit one or more clocks to selectively drive the clocked inverter.
 6. A digital logic circuit comprising a plurality of flip-flops with at least one of the plurality having an architecture according to claim
 1. 7. The circuit according to claim 6, wherein the at least one flip-flop is connected to another flip-flop to provide scan mode testing, the connection being free of hold violations.
 8. The architecture according to claim 1, wherein the selection signal is operable to select between a functional mode and a scan mode, the flip-flop being edge-triggered in functional mode.
 9. The architecture according to claim 1, wherein one of the clocking signals is a functional mode clock signal for clocking the master stage in a functional mode.
 10. The architecture according to claim 9, further comprising a logic gate coupled to the selection signal and the functional mode clock signal to prevent the functional mode clock signal from being applied to the master stage based on a state of the selection signal.
 11. The architecture according to claim 10, wherein the state of the selection signal permits flip-flop operation as back-to-back master and slave latches.
 12. The architecture according to claim 11, wherein: the state of the selection signal permits the master stage to be clocked by another of the plurality of clocking signals; and the state of the selection signal permits the slave stage to be clocked by the functional mode clock signal.
 13. The circuit according to claim 7, wherein the connection is free of delay buffers.
 14. The architecture according to claim 1, wherein the flip-flop is operable in a scan mode, wherein: the plurality of clocking signals includes a scan clock signal; a state of the selection signal permits the master stage to be clocked with the scan clock signal; the state of the selection signal permits the slave stage to be clocked with a functional clock signal; and the scan clock signal and the functional clock signal are non-overlapping.
 15. The architecture according to claim 14, wherein an amount of non-overlap can be adjusted to attain desired parameter settings.
 16. The architecture according to claim 1, wherein the flip-flop is externally compatible with operation of prior flip-flop architectures.
 17. The circuit according to claim 7, wherein the connection is free of lockup latches.
 18. A method for operating a flip-flop, comprising: selecting a test mode for testing flip-flop operation; clocking a master stage of the flip-flop with a first clock signal during test mode; selecting a normal functional mode for normal functional operation of the flip-flop; and clocking the master stage of the flip-flop with a second clock signal during normal mode operation.
 19. The method according to claim 18, further comprising supplying the first and second clock signals on separate respective connections to the master stage.
 20. The method according to claim 18, further comprising selectively applying the first or second clock signals to a master stage clocked inverter to provide selective clocked operation of the clocked inverter.
 21. The method according to claim 18, further comprising selectively enabling a flip-flop output in scan mode.
 22. A flip flop architecture, comprising: a scan mode data path clocked by a scan mode clock; a functional data path clocked by a functional clock; a master stage including a clocked inverter selectively clocked with the functional mode clock or the scan mode clock; a slave stage coupled to the master stage and clocked with the functional mode clock; and a selection signal coupled to at least one of the clock signals to selectively disable the clock signal to the master stage.
 23. The architecture according to claim 22, further comprising a scan mode output enable driven by the selection signal.
 24. The architecture according to claim 22, wherein the scan mode clock and the functional mode clock are non-overlapping when scan mode is selected.
 25. The architecture according to claim 22, wherein the functional data path is edge triggered.
 26. A digital logic circuit comprising a plurality of flip-flops with at least one of the plurality having an architecture according to claim
 22. 27. The circuit according to claim 26, wherein the at least one of the flip-flops is connected to another flip-flop to provide scan mode testing, the connection being free of hold violations.
 28. The circuit according to claim 27, wherein the connection is free of delay buffers.
 29. The architecture according to claim 22, wherein the flip-flop is externally compatible with operation of prior flip-flop architectures.
 30. The circuit according to claim 27, wherein the connection is free of lockup latches.
 31. A clock signal generator for generating clock signals for a flip-flop, comprising: a plurality of clock manipulation circuits, each being operable to receive a clock signal input and generate a scan clock signal; enable circuitry coupled to at least one of the plurality of clock manipulation circuits and operable to enable the at least one clock manipulation circuit during a scan mode; and the scan clock signals generated by the plurality of clock manipulation circuits being non-overlapping clock signals.
 32. The generator according to claim 31, wherein the enable circuitry permits the clock signal input to be provided on an output of at least one of the clock manipulation circuits during a functional mode.
 33. The generator according to claim 31, wherein the enable circuitry provides a gating operation on the clock signal input to permit the scan clock signals to be independent of the clock signal input during the scan mode.
 34. A method for generating scan mode clock signals for a flip-flop, comprising: selectively gating an input clock signal to gate every other clock pulse to one of two different destinations; producing a first and second scan clock signal based on the every other clock pulses received at the two different destinations; and arranging the gating and the input clock signal to permit the first and second scan clock signals to be non-overlapping.
 35. The method according to claim 34, further comprising enabling the selective gating based on a scan mode indication.
 36. A method for generating scan mode clock signals for a flip-flop, comprising: forming a first scan mode clock signal based on leading edges of an input clock signal; forming a second scan mode clock signal based on trailing edges of the input clock signal; and arranging the first and second scan mode clock signals to be non-overlapping.
 37. The method according to claim 36, further comprising enabling formation of the first and second scan mode clock signals based on a scan mode indication.
 38. The method according to claim 36, wherein one of the first or second scan mode clock signals is an inversion of the input clock signal.
 39. The method according to claim 36, wherein forming the first or second scan mode clock signal further comprises generating a pulse based on the leading or trailing edge of the input clock signal, respectively.
 40. The method according to claim 36, wherein the input clock signal is a functional mode clock signal. 